As the statistics shows, a constantly growing demand of people for a data service rate and capacity has failed to be accommodated in a single-layer coverage network consisted of traditional macro eNBs, so that layered network deployment has been introduced in the 3rd Generation Partnership Project (3GPP) to address this problem, that is, some low-power eNBs, i.e., local eNBs (including femto eNBs, pico eNBs, relay nodes or other forms) are deployed in a hotspot area, a home indoor environment, an office environment or other small-coverage environments to thereby split a cell so as to enable an operator to provide a user with a service at a higher data rate and a lower cost.
FIG. 1 illustrates a network architecture of an Evolved Universal Terrestrial Radio Access Network (E-UTRNA) consisted of eNodeBs (eNBs), Home eNodeBs (HeNBs), etc.
A Mobility Management Entity (MME) is connected with an eNB via an S1-MME interface; and the eNB functions as an access network and communicates with a User Equipment (UE) via an air interface. Each UE attached to the network is served by an MME which is referred to as a serving MME of the UE. The S1-MME interface provides the UE with control plane services including mobility management and bearer management functions.
A HeNB can be connected directly with a core network node MME or can access the network through a HeNB Gateway (GW) node. Both the HeNB GW and the eNB are equivalent from the perspective of the MME, and both the HeNB GW and the MME are equivalent from the perspective of the HeNB.
An SGW is connected with an eNB via an S1-U interface, and each UE attached to the network is served by an SGW which is referred to as a serving SGW of the UE. The S1-U interface provides the UE with user plane services, and user plane data of the UE is transmitted between the SGW and the eNB over an S1-U General Packet Radio Service (GPRS) Tunneling Protocol (GTP) bearer.
The S1-U interface of the HeNB can be terminated at the HeNB GW, or the S1-U interface can be set up directly between the HeNB and the SGW.
An X2 interface can be set up between HeNBs, and an X2 Interface can also be set up between the HeNB and the macro eNB.
Mobility scenario of the X2 interface supported by the HeNB are as depicted in Table 1 below:
TABLE 1SourceTargetNoteseNB or any HeNBOpen access HeNBeNB or any HeNBHybrid access HeNBHybrid access HeNBClosed access HeNBOnly applies for sameor closed accessClosed SubscriberHeNBGroup (CSG) ID andPublic Land MobileNetwork (PLMN),and if the UE is amember of the CSGcell.)Any HeNBeNB
In order to lower the frequency at which the UE is handed over between a macro eNB cell and a local eNB cell, a network deployment scheme in which the user plane is separated from the control plane has been introduced as illustrated in FIG. 2.
In this scheme, when the UE resides in an area covered only by the macro eNB cell, both control plane connections and user plane connections of the UE are active at the macro eNB; and when the UE is moved to an area covered by both the macro eNB cell and the local eNB cell, all or a part of the user plane connections (i.e., Data Radio Bearers (DRBs)) of the UE are transferred to the local eNB for a higher service transmission rate, and most of the control plane connections remain active at the macro eNB to thereby prevent a call of the UE from being dropped due to a handover failure of a control plane connection.
FIG. 3 illustrates a handover procedure of the UE in the traditional LTE network:
Operation 1. The source eNB configures the UE to make measurement, and the UE makes measurement according to received measurement configuration information;
Operation 2. The UE reports a measurement result over an uplink resource allocated by the source eNB for the UE, to assist the source eNB in making a handover decision;
Operation 3. The source eNB makes a handover decision. If the source eNB decides to perform handover, then the flow proceeds to the following operations;
Operation 4. The source eNB transmits a handover request message carrying handover preparation related information to the target eNB;
Operation 5. The target eNB refers to the handover request message and makes an admission decision according to QoS information of a bearer to be admitted. If the target eNB allows the UE to be admitted, then the target eNB configures a lower layer to prepare for handover;
Operation 6. The target eNB returns a handover request response message including an RRC container, which is particularly a handover command to trigger the handover of the UE, to the source eNB;
Operation 7. The source eNB transmits the received handover command transparently (without any modification thereto) to the UE over a scheduled downlink resource; and the UE stops data from being transmitted and received with the source eNB upon reception of the handover command;
Operation 8. The source eNB transmits sequence number (SN) status information of ongoing data transmission (e.g., a sequence number of an unsuccessfully transmitted downlink data packet, a first sequence number which can be allocated by the target eNB, etc.) to the target eNB so that sequence numbers of data transmitted and received by the source eNB and the target eNB are consecutive to thereby avoid any packet from being lost or retransmitted;
Subsequent to the operation 8, the source eNB can transmit its downlink data packets, received from the core network but unsuccessfully transmitted to the UE, to the target eNB so that the target eNB transmits them to the UE; and alike the source eNB can transmit uplink data packets of the UE, received via the air interface, with inconsecutive sequence numbers (a packet transmitted later by the UE may be received successfully by the DeNB before a packet transmitted earlier is received successfully by the DeNB due to an error in transmission via the air interface so that there may be inconsecutive sequence numbers of the packets received by the DeNB) to the target eNB so that the target eNB transmits the data packets with consecutive sequence numbers to the core network upon reception of absent data packets retransmitted by the UE.
Operation 9. The UE transmits a preamble to the target eNB for uplink synchronization with the target eNB;
Operation 10. The target eNB returns a response message carrying an uplink resource allocated for the UE (for subsequent transmission) and a timing advance of the UE (to adjust uplink transmission instance of time of the UE for uplink synchronization);
Operation 11. The UE returns a handover completion message to the target eNB;
Thereafter data can be transmitted and received between the UE and the target eNB.
Operation 12. The target eNB transmits to the MME a path switch request carrying user plane transport layer addresses (IP addresses) and downlink GTP tunnel identifiers (Tunnel Endpoint Identifiers (TEIDs)) specified by the target eNB respectively for respective Evolved Packet System (EPS) bearers of the UE;
Operation 13. The MME transmits to the SGW a bearer modification request carrying the user plane transport layer addresses (IP addresses) and the downlink GTP tunnel identifiers (TEIDs) specified by the target eNB respectively for the respective EPS bearers of the UE;
Operation 14. The SGW switches the path;
Operation 15. The SGW returns to the MME a bearer modification response carrying user plane transport layer addresses (IP addresses) and uplink GTP tunnel identifiers (TEIDs) specified by the SGW respectively for the respective EPS bearers of the UE;
Operation 16. The MME returns to the target eNB a path switch response carrying the user plane transport layer addresses (IP addresses) and the uplink GTP tunnel identifiers (TEIDs) specified by the SGW respectively for the respective EPS bearers of the UE;
The path has been switched so far. Thereafter for a downlink data packet transmitted to the UE, the SGW can transmit it to the corresponding user plane transport layer address (IP address) specified by the target eNB, and set the downlink GTP tunnel identifier (TEID), according to a bearer to which it belongs; and for an uplink data transmitted by the UE, the target eNB can transmit it to the corresponding user plane transport layer address (IP address) specified by the SGW, and set the uplink GTP tunnel identifier (TEID), according to a bearer to which it belongs;
Operation 17. The target eNB transmits a UE context release request to the source eNB;
Operation 18. The source eNB releases the related resources allocated for the handed over UE.
FIG. 4 illustrates a flow chart of requesting by the UE for a service:
Operations 1 to 3: The UE initiates a Radio Resource Control (RRC) connection setup procedure to the cell where it resides;
Operation 4: After an RRC connection is set up, the serving eNB initiates an S1 signaling setup procedure to the MME by transmitting S1 signaling which is an initial UE message carrying a Non-Access Stratum (NAS) service request message;
Operation 5. The MME trigger an initial context setup procedure of the UE upon reception of the initial UE message by transmitting an initial context setup request message carrying information about an Evolved Radio Access Bearer (E-RAB) to be set up, e.g., Quality of Service (QoS) information, a transport layer address, etc.;
Operation 6. The serving eNB performs access control according to the QoS information of the E-RAB;
Operation 7. If the serving eNB allows the UE to access, then the serving eNB generates an RRC reconfiguration message and transmits it to the UE;
Operation 8. The UE reconfigures all the DRB resources and subsequently transmits an RRC reconfiguration completion message to the serving eNB;
Operation 9. The serving eNB transmits an initial context response message carrying a downlink transport layer address of the serving eNB to the MME; and
Operations 10 to 11: The MME updates the bearer with the SGW. Thereafter the UE starts transmission of uplink and downlink data with the network.
There has been absent in the prior art a process of transferring a user plane bearer when a target local eNB is a Closed Subscriber Group (CSG) local eNB. If the process of transferring a bearer is applied where the target local eNB is a macro eNB, then a user plane bearer may fail to be transferred, thus degrading reception and transmission by the UE of the service, because when the CSG local eNB operates in the hybrid access mode, only a UE belonging to a CSG (which can be any CSG) can be allowed to access, and if a bearer of a UE which does not belong to the CSG is transferred to the CSG local eNB, then the UE bearer may fail to be transferred; and if the eNB operates in the closed access mode, then only a UE belonging to a CSG of the CSG local eNB can be allowed to access, and if a bearer of a UE which does not belong to the CSG of the CSG local eNB is transferred to the CSG local eNB, then the UE bearer may fail to be transferred.